The desirable properties of aluminum and its alloys such as low cost, low density, corrosion resistance, and ease of fabrication are well known.
One important means for enhancing the strength of aluminum alloys is heat treatment. Conventionally, three basic steps are employed in the heat treatment of aluminum alloys: (1) Solution heat treating; (2) Quenching; and (3) Aging. Additionally, a cold working step is often added prior to aging. Solution heat treating consists of soaking the alloy at a temperature sufficiently high and for a long enough time to achieve a nearly homogeneous solid solution of precipitate-forming elements in aluminum. The objective is to take into solid solution the maximum practical amounts of the soluble hardening elements. Quenching involves the rapid cooling of the solid solution, formed during the solution heat treatment, to produce a supersaturated solid solution at room temperature. The aging step involves the formation of strengthening precipitates from the rapidly cooled supersaturated solid solution. Precipitates may be formed using natural (ambient temperature), or artificial (elevated temperature) aging techniques. In natural aging, the quenched alloy is held at temperatures in the range of -20.degree. to +50.degree. C., typically at room temperature, for relatively long periods of time. For certain alloy compositions, the precipitation hardening that results from natural aging alone produces useful physical and mechanical properties. In artificial aging, the quenched alloy is held at temperatures typically in the range of 100.degree. to 200.degree. C. for periods of approximately 5 to 48 hours, typically, to effect precipitation hardening.
The extent to which the strength of Al alloys can be increased by heat treatment is related to the type and amount of alloying additions used. The addition of copper to aluminum alloys, up to a certain point, improves strength, and in some instances enhances weldability. The further addition of magnesium to Al-Cu alloys can improve resistance to corrosion, enhance natural aging response without prior cold work and increase strength. However, at relatively low Mg levels, weldability is decreased.
One commercially available aluminum alloy containing both copper and magnesium is alloy 2024, having nominal composition Al - 4.4 Cu - 1.5 Mg - 0.6 Mn. Alloy 2024 is a widely used alloy with high strength, good toughness, good warm temperature properties and a good natural- aging response. However, its corrosion resistance is limited in some tempers, it does not provide the ultrahigh strength and exceptionally strong natural-aging response achievable with the alloys of the present invention, and it is only marginally weldable. In fact, 2024 welded joints are not considered commercially useful in most situations.
Another commercial Al-Cu-Mg alloy is alloy 2519 having a nominal composition of Al - 5.6 Cu - 0.2 Mg - 0.3 Mn - 0.2 Zr - 0.06 Ti - 0.05 V. This alloy was developed by Alcoa as an improvement on 2219, which is presently used in various aerospace applications. While the addition of Mg to the Al-Cu system can enable a natural-aging response without prior cold work, 2519 has only marginally improved strengths over 2219 in the highest strength tempers.
Work reviewed by Mondolfo on conventional Al-Cu-Mg alloys indicates that the main hardening agents are CuAl.sub.2 type precipitates in alloys in which the Cu to Mg ratio is greater than 8 to 1 (See ALUMINUM ALLOYS: STRUCTURE AND PROPERTIES, L.F. Mondolfo, Boston: Butterworths, 1976, p. 502).
Polmear, in U.S. Pat. No. 4,772,342, has added silver and magnesium to the Al-Cu system in order to increase elevated temperature properties. A preferred alloy has the composition Al - 6.0 Cu - 0.5 Mg - 0.4 Ag - 0.5 Mn - 0.15 Zr - 0.10 V - 0.05 Si. Polmear associates the observed increase in strength with the "omega phase" that arises in the presence of Mg and Ag (see "Development of an Experimental Wrought Aluminum Alloy for Use at Elevated Temperatures," Polmear, ALUMINUM ALLOYS: THEIR PHYSICAL AND MECHANICAL PROPERTIES, E.A. Starke, Jr. and T.H. Sanders, Jr., editors, Volume I of Conference Proceedings of International Conference, University of Virginia, Charlottesville, Va., Jun. 15-20, 1986, pages 661-674, Chameleon Press, London).
Adding lithium to Al-Mg alloys and to Al-Cu alloys is known to lower the density and increase the elastic modulus, producing significant improvements in specific stiffness and enhancing the artificial age hardening response. However, conventional Al-Li alloys generally possess relatively low ductility at given strength levels and toughness is often lower than desired, thereby limiting their use.
Difficulties in melting and casting have limited the acceptance of Al-Li alloys. For example, because Li is extremely reactive, Al-Li melts can react with the refractory materials in furnace linings. Also, the atmosphere above the melt has to be controlled to reduce oxidation problems. In addition, lithium lowers the thermal conductivity of aluminum, making it more difficult to remove heat from an ingot during direct-chill casting, thereby decreasing casting rates. Furthermore, in Al-Li melts containing 2.2 to 2.7 percent Lithium, typical of recently commercialized Al-Li alloys, there is considerable risk of explosion. To date, the property benefits attributable to these new Al-Li alloys have not been sufficient to offset the increase in processing costs caused by the above-mentioned problems. As a consequence they have not been able to replace conventional alloys such as 2024 and 7075. The preferred alloys of the present invention do not create these melting and casting problems to as great a degree because of their lower Li content.
Al-Li alloys containing Mg are well known, but they typically suffer from low ductility and low toughness. One such system is the low density, weldable Soviet alloy 01420 as disclosed in British Patent 1,172,736, to Fridlyander et al, of nominal composition Al - 5 Mg - 2 Li.
Al-Li alloys containing Cu are also well known, such as alloy 2020, which was developed in the 1950's, but was withdrawn from production because of processing difficulties and low ductility. Alloy 2020 falls within the range disclosed in U.S. Pat. No. 2,381,219 to LeBaron, which emphasizes that the alloys are "magnesium-free", i.e. the alloys have less than 0.01 percent Mg, which is present only as an impurity. In addition, the alloys disclosed by LeBaron require the presence of at least one element selected from Cd, Hg, Ag, Sn, In and Zn. Alloy 2020 has relatively low density, good exfoliation corrosion resistance and stress-corrosion cracking resistance, and retains a useful fraction of its strength at slightly elevated temperatures. However, it suffers from low ductility and low fracture toughness properties in high strength tempers, thereby limiting its usefulness.
To achieve the highest strengths in Al-Cu-Li alloys, it is necessary to introduce a cold working step prior to aging, typically involving rolling and/or stretching of the material at ambient or near ambient temperatures. The strain which is introduced as a result of cold working produces dislocations within the alloy which serve as nucleation sites for the strengthening precipitates. In particular, conventional Al-Cu-Li alloys must be cold worked before artificial aging in order to obtain high strengths, i.e. greater than 70 ksi ultimate tensile strength (UTS). Cold working of these alloys is necessary to promote high volume fractions of Al.sub.2 CuLi (T.sub.1) and Al.sub.2 Cu (theta-prime) precipitates which, due to their high surface-to-volume ratio, nucleate far more readily on dislocations than in the aluminum solid solution matrix. Without the cold working step, the formation of the plate-like Al.sub.2 CuLi and Al.sub.2 Cu precipitates is retarded, resulting in significantly lower strengths. Moreover, the precipitates do not easily nucleate homogeneously because of the large energy barrier that has to be overcome due to their large surface area. Cold working is also useful, for the same reasons, to produce the highest strengths in many commercial Al-CU alloys, such as 2219.
The requirement for cold working to produce the highest strengths in Al-Cu-Li alloys is particularly limiting in forgings, where it is often difficult to uniformly introduce cold work to the forged part after solutionizing and quenching. As a result, forged Al-Cu-Li alloys are typically limited to non-cold worked tempers, resulting in generally unsatisfactory mechanical properties.
Recently, Al-Li alloys containing both Cu and Mg have been commercialized. These include alloys 8090, 2091, 2090, and CP 276. Alloy 8090, as disclosed in U.S. Pat. No. 4,588,553 to Evans et al, contains 1.0-1.5 Cu, 2.0-2.8,Li, and 0.4-1.0 Mg. The alloy was designed with the following properties for aircraft applications: good exfoliation corrosion resistance, good damage tolerance, and a mechanical strength greater than or equal to 2024 in T3 and T4 conditions. Alloy 8090 does exhibit a natural aging response without prior cold work, but not nearly as strong as that of the alloys of the present invention. In addition, 8090-T6 forgings have been found to possess a low transverse elongation of 2.5 percent.
Alloy 2091, with 1.5-3.4 Cu, 1.7-2.9 Li, and 1.2-2.7 Mg, was designed as a high strength, high ductility alloy. However, at heat treated conditions that produce maximum strength, ductility is relatively low in the short transverse direction.
In recent work on alloys 8090 and 2091, Marchive and Charue have reported reasonably high longitudinal tensile strengths (see "Processing and Properties 4TH INTERNATIONAL ALUMINIUM LITHIUM CONFERENCE, G. Champier, B. Dubost, D. Miannay, and L. Sabetay editors, Proceedings of International Conference, Jun. 10-12, 1987, Paris, France, pp. 43-49). In the T6 temper, 8090 possesses a yield strength of 67.3 ksi and an ultimate tensile strength of 74 ksi, while 2091 possesses a yield strength of 63.8 ksi and an ultimate tensile strength of 75.4 ksi. However, the strengths of both 8090-T6 and 2091-T6 forgings are still below those obtained in the T8 temper, e.g. for 8090-T851 extrusions, tensile properties are 77.6 ksi YS and 84.1 ksi UTS, while for 2091-T851 extrusions, tensile properties are 73.3 ksi YS and 84.1 ksi UTS. By contrast, the Al-Cu-Li-Mg alloys of the present invention possess highly improved properties compared to conventional 8090 and 2091 alloys in both cold worked and non-cold worked tempers.
Alloy 2090, which may contain only minor amounts of Mg, comprises 2.4-3.0 Cu, 1.9-2.6 Li and 0-0.25 Mg. The alloy was designed as a low-density replacement for high strength products such as 2024 and 7075. However, it has weldment strengths that are lower than those of conventional weldable alloys such as 2219 which possesses weld strengths of 35-40 ksi. As cited in the following reference, in the T6 temper alloy 2090 cannot consistently meet the strength, toughness, and stress-corrosion cracking resistance of 7075-T73 (see "First Generation Products- 2090, " Bretz, ALITHALITE ALLOYS: 1987 UPDATE, J. Kar, S.P. Agrawal, W.E. Quist, editors, Conference Proceedings of International Aluminum-Lithium Symposium, Los Angeles, Calif., Mar. 25-26, 1987, pages 1-40). As a consequence, the properties of current Al-Cu-Li alloy 2090 forgings are not sufficiently high to justify their use in place of existing 7XXX forging alloys.
It should be noted that the addition of Mg to the Al-Cu-Li system does not in its own right cause an increase in alloy strength in high strength tempers. For example alloy 8090 (nominal composition Al - 1.3 Cu - 2.5 Li - 0.7 Mg) does not have significantly greater strength compared to nominally Mg-free alloy 2090 (nominal composition Al - 2.7 Cu - 2.2 Li - 0.12 Zr). Furthermore, Mg-free alloy 2020 of nominal composition Al - 4.5 Cu - 1.1 Li - 0.4 Mn - 0.2 Cd is even slightly stronger than Mg containing alloy 8090.
Several patent documents relating to Al-Cu-Li-Mg alloys exist. European Patent No. 158,571 to Dubost, assigned to Cegedur Societe de Transformation de l'Aluminum Pechiney, relates to Al alloys comprising 2.75-3.5 Cu, 1.9-2.7 Li, 0.1-0.8 Mg, balance Al and grain refiners. The alloys, which are commercially known as CP 276, are said to possess high mechanical strength combined with a decrease in density of 6-9 percent compared with conventional 2xxx (Al-Cu) and 7xxx (Al-Zn-Mg) alloys. The compositional ranges disclosed by Dubost are outside of the ranges of the present invention. Specifically, Dubost's Li content is higher than the Li content of the alloys of the present invention containing less than about 5 percent Cu. Such high levels of Li are required by Dubost in order to lower density over that of conventional alloys. In addition, the maximum Cu level of 3.5 percent given by Dubost is below the preferred Cu level of the present invention. Limiting Cu content to a maximum of 3.5 percent also serves to minimize density in the alloys of Dubost. While Dubost lists high yield strengths of 498-591 MPa (72-85 ksi) for his alloys in the T6 condition, the elongations achieved are relatively low (2.5-5.5 percent).
U.S. Pat. No. 4,752,343 to Dubost et al, assigned to Cegedur Sodiete de Transformation de l'Aluminum Pechiney, relates to Al alloys comprising 1.5-3.4 Cu, 1.7-2.9 Li, 1.2-2.7 Mg , balance Al and grain refiners. The ratio of Mg to Cu must be between 0.5 and 0.8. The alloys are said to possess mechanical strength and ductility characteristics equivalent to conventional 2xxx and 7xxx alloys. The compositional ranges disclosed by Dubost et al are outside of the ranges of the present invention. For example, the maximum Cu content listed by Dubost et al is lower than the minimum Cu level of the present invention. Additionally, the minimum Mg content of Dubost et al is higher than the maximum Mg level permitted in the present alloys containing less than about 5 percent Cu. Further, the minimum Mg to Cu ratio of 0.5 permitted by Dubost et al is far above the Mg/Cu ratio of the present alloys. While the purpose of Dubost et al is to produce alloys having mechanical strengths and ductilities comparable to conventional alloys, such as 2024 and 7475, the actual strength/ ductility combinations achieved are below those attained by the alloys of the present invention.
U.S. Pat. No. 4,652,314 to Meyer, assigned to Cegedur Societe de Transformation de l'Aluminum Pechiney, is directed to a method of heat treating Al-Cu-Li-Mg alloys. The process is said to impart a high level of ductility and isotropy in the final product. While Meyer teaches that his heat treating method is applicable to Al-Cu-Li-Mg alloys, the specific compositions disclosed by Meyer are outside of the compositional ranges of the present invention. Also, the properties which Meyer achieves are below those of the present invention. For example, the highest yield strength achieved by Meyer is 504 MPa (73 ksi) for a cold worked, artificially aged alloy in the longitudinal direction, which is significantly below the yield strengths attained in the alloys of the present invention in the cold worked, artificially aged condition.
U.S. Pat. No. 4,526,630 to Field, assigned to Alcan International Ltd., relates to a method of heat treating Al-Li alloys containing Cu and/or Mg. The process, which constitutes a modification of conventional homogenization techniques, involves heating an ingot to a temperature of at least 530.degree. C. and maintaining the temperature until the solid intermetallic phases present within the alloy enter into solid solution. The ingot is then cooled to form a product which is suitable for further thermomechanical treatment, such as rolling, extrusion or forging. The process disclosed is said to eliminate undesirable phases from the ingot, such as the coarse copper-bearing phase present in prior art Al-Li-Cu-Mg alloys. Field teaches that his homogenization treatment is limited to Al-Li alloys having compositions within specified ranges. For known Al-Li-Cu-Mg based alloys, compositions are limited to 1-3 Li, 0.5-2 Cu, and 0.2-2 Mg. For conventional Al-Li-Mg based alloys, compositions are limited to 1-3 Li, 2-4 Mg, and below 0.1 Cu. For known Al-Li-Cu based alloys, compositions are limited to 1-3 Li, 0.5-4 Cu, and up to 0.2 Mg. The alloys of the present invention do not fall within any of these compositional ranges disclosed by Field. Furthermore, the present alloys possess superior properties, such as increased strength, compared to the properties disclosed by Field.
The following references disclose additional Al, Cu, Li and Mg containing alloys having compositional ranges that are outside of the ranges of the present invention: U.S. Pat. No. 3,306,717 to Lindstrand et al; U.S. Pat. No. 3,346,370 to Jagaciak et al; U.S. Pat. No. 4,584,173 to Gray et al; U.S. Pat. No. 4,603,029 to Quist et al; U.S. Pat. No. 4,626,409 to Miller; U.S. Pat. No. 4,661,172 to Skinner et al; U.S. Pat. No. 4,758,286 to Dubost et al; European Patent Application Publication No. 0188762 to Hunt et al; European Patent Application Publication No. 0149193; Japanese Pat. No. J6-0238439; Japanese Pat. No. J6-1133358; and Japanese Pat. No. J6-1231145.
There are a limited number of references relating to Al-Cu-Li-Mg alloys that disclose amounts of Cu of to 5 percent. None of these references disclose the specific alloy compositions of the present invention, nor do they disclose the highly desirable properties which the alloys of the present invention have been found to possess. In addition, none of these references disclose the necessity of the high Cu to Li ratio required in the alloys of the present invention. While each of the-following references disclose broad ranges of Cu, Li and Mg that may be alloyed with Al, none of these references disclose the critical ranges and combinations of Cu, Li and Mg of the present invention which produce alloys having physical and mechanical properties that heretofore have not been achieved.
U.S. Pat. No. 4,648,913 to Hunt et al, assigned to Alcoa, relates to a method of cold working Al-Li alloys wherein solution heat treated and quenched alloys are subjected to greater than 3 percent stretch at room temperature. The alloy is then artificially aged to produce a final alloy product. The cold work imparted by the process of Hunt et al is said to increase strength while causing little or no decrease in fracture toughness of the alloys. The particular alloys utilized by Hunt et al are chosen such that they are responsive to the cold working and aging treatment disclosed. That is, the alloys must exhibit improved strength with minimal loss in fracture toughness when subjected to the cold working treatment recited (greater than 3 percent stretch) in contrast to the result obtained with the same alloy if cold worked less than 3 percent. Hunt et al broadly recite ranges of alloying elements which, when combined with Al, may produce alloys that are responsive to greater than 3 percent stretch. The disclosed ranges are 0.5-4.0 Li, 0-5.0 Mg, up to 5.0 Cu, 0-1.0 Zr, 0-2.0 Mn, 0-7.0 Zn, balance Al. While Hunt et al disclose very broad ranges of several alloying elements, there is only a limited range of alloy compositions that would actually exhibit the required combination of improved strength and retained fracture toughness when subjected to greater than 3 percent stretch. Particularly, the alloy compositions of the present invention do not exhibit the response to cold working which is required by Hunt et al. Rather, the strengths achieved in the alloys of the present invention remain substantially constant when subjected to varying amounts of stretch. Thus, the alloys of the present invention are distinct from, and possess advantages over, the alloys contemplated by Hunt et al, because large amounts of cold work are not required to achieve improved properties. In addition, the yield strengths which Hunt et al achieve in the alloy compositions disclosed are substantially below those which are attained in the alloys of the present invention. Further, Hunt et al indicate that it is preferred in their process to artificially age the alloy after cold working, rather than to naturally age. In contrast, the alloys of the present invention exhibit an extremely strong natural aging response, providing high elongations and only slightly lower strengths than in the artificially aged tempers.
U.S. Pat. No. 4,795,502 to Cho, assigned to Alcoa, is directed to a method of producing unrecrystallized wrought Al-Li sheet products having improved levels of strength and fracture toughness. In the process of Cho, a homogenized aluminum alloy ingot is hot rolled at least once, cold rolled, and subjected to a controlled reheat treatment. The reheated product is then solution heat treated, quenched, cold worked to induce the equivalent of greater than 3 percent stretch, and artificially aged to provide a substantially unrecrystallized sheet product having improved levels of strength and fracture toughness. The final product is characterized by a highly worked microstructure which lacks well-developed grains. The Cho reference appears to be a modification of the Hunt et al reference listed above, in that a controlled reheat treatment is added prior to solution heat treatment which prevents recrystallization in the final product formed. Cho discloses that aluminum base alloys within the following compositional ranges are suitable for the recited process: 1.6-2.8 Cu, 1.5-2.5 Li, 0.7-2.5 Mg, and 0.03-0.2 Zr. These ranges are outside of the compositional ranges of the present invention. For example, the maximum Cu level of 2.8 percent listed by Cho is well below the minimum Cu level of the present invention. However, Cho then goes on to broadly state that the alloy of his invention can contain 0.5-4.0 Li, 0-5.0 Mg, up to 5.0 Cu, 0-1.0 Zr, 0-2.0 Mn, and 0-7.0 Zn. As in the Hunt et al reference, the particular alloys utilized by Cho are apparently chosen such that they exhibit a combination of improved strength and fracture toughness when subjected to greater than 3 percent cold work. The alloys of Cho must further be susceptible to the reheat treatment recited. As discussed above, the alloys of the present invention attain essentially the same ultra-high strength with varying amounts of stretch and do not require cold work to obtain their extremely high strengths. While Cho provides a process which is said to improve strength in known Al-Li alloys, such as 2091, the strengths attained are substantially below those achieved in the alloys of the present invention. Cho also indicates that artificial aging should be used in his process to obtain advantageous properties. In contrast, the alloys of the present invention do not require artificial aging. Rather, the present alloys exhibit an extremely strong natural aging response which permits their use in applications where artificial aging is impractical. It can therefore be seen that the alloys of the present invention are distinct from the alloys amenable to the process taught by Cho.
European Patent Application No. 227,563, to Meyer et al, assigned to Cegedur Societe de Transformation de l'Aluminum Pechiney, relates to a method of heat treating conventional Al-Li alloys to improve exfoliation corrosion resistance while maintaining high mechanical strength. The process involves the steps of homogenization, extrusion, solution heat treatment and cold working of an Al-Li alloy, followed by a final tempering step which is said to impart greater exfoliation corrosion resistance to the alloy, while maintaining high mechanical strength and good resistance to damage. Alloys subjected to the treatment have a sensitivity to the EXCO exfoliation test of less than or equal to EB (corresponding to good behavior in natural atmosphere) and a mechanical strength comparable with 2024 alloys. Meyer et al list broad ranges of alloying elements which, when combined with Al, can produce alloys that may be subjected to the final tempering treatment disclosed. The ranges listed include 1-4 Li, 0-5 Cu, and 0-7 Mg. While the reference lists very broad ranges of alloying elements, the actual alloys which Meyer et al utilize are the conventional alloys 8090, 2091, and CP276. Thus, Meyer et al do not teach the formation of new alloy compositions, but merely teach a method of processing known Al-Li alloys. The highest yield strength achieved in accordance with the process of Meyer et al is 525 MPa (76 ksi) for alloy CP276 (2.0 Li, 3.2 Cu, 0.3 Mg, 0.11 Zr, 0.04 Fe, 0.04 Si, balance Al) in the cold worked, artificially aged condition. This maximum yield strength listed by Meyer et al is below the yield strengths achieved in the alloys of the present invention in the cold worked, artificially aged condition. In addition, the final tempering method of Meyer et al is said to improve exfoliation corrosion resistance in Al-Li alloys, whereby sensitivity to the EXCO exfoliation corrosion test is improved to a rating of less than or equal to EB. In contrast, the alloys of the present invention possess an exfoliation corrosion resistance rating of less than or equal to EB without the use of a final tempering step. The present alloys are therefore distinct from, and advantageous over, the alloys contemplated by Meyer et al, because a final tempering treatment is not required in order to achieve favorable exfoliation corrosion properties.
U.K. Patent Application No. 2,134,925, assigned to Sumitomo Light Metal Industries Ltd., is directed to Al-Li alloys having high electrical resistivity. The alloys are suitable for use in structural applications, such as linear motor vehicles and nuclear fusion reactors, where large induced electrical currents are developed. The primary function of Li in the alloys of Sumimoto is to increase electrical resistivity. The reference lists broad ranges of alloying elements which, when combined with Al, may produce structural alloys having increased electrical resistivity. The disclosed ranges are 1.0-5.0 Li, one or more grain refiners selected from Ti, Cr, Zr, V and W, and the balance Al. The alloy may further include 0-5.0 Mn and/or 0.05-5.0 Cu and/or 0.05-8.0 Mg. Sumitomo discloses particular Al-Li-Cu and Al-Li-Mg based alloy compositions which are said to possess the improved electrical properties. In addition, Sumitomo discloses one Al-Li-Cu-Mg alloy of the composition 2.7 Li, 2.4 Cu, 2.2 Mg, 0.1 Cr, 0.06 Ti, 0.14 Zr, balance aluminum, which possesses the desired increase in electrical resistivity. The Li and Cu levels given for this alloy are outside of the Li and Cu ranges of the present invention. Additionally, the Mg level given is outside of the preferred Mg range of the present invention. The strengths disclosed by Sumitomo are far below those achieved in the present invention. For example, in the Al-Li-Cu based alloys listed, Sumitomo gives tensile strengths of about 17-35 kg/mm2 (24-50 ksi). In the Al-Li-Mg based alloys listed, Sumitomo discloses tensile strengths of about 43-52 kg/mm2 (61-74 ksi). It is desired in Sumitomo to produce alloys having the highest possible strengths in order to produce alloys for the structural applications disclosed. However, since the strengths actually achieved in the reference are well below the strengths attained in the alloys of the present invention, it is evident that Sumitomo has neither discovered nor considered the specific alloys of the present invention.
It should be noted that prior art Al-Cu-Li-Mg alloys have almost invariably limited the amount of Cu to 5 weight percent maximum due to the known detrimental effects of higher Cu content, such as increased density. According to Mondolfo, amounts of Cu above 5 weight percent do not increase strength, tend to decrease fracture toughness, and reduce corrosion resistance (Mondolfo, pp. 706-707.) These effects are thought to arise because in Al-Cu engineering alloys, the practical solid solubility limit of Cu is approximately 5 weight percent, and hence any Cu present above about 5 weight percent forms the less desired primary theta-phase. Moreover, Mondolfo states that in the quaternary system Al-Cu-Li-Mg the Cu solubility is further reduced. He concludes, "The solid solubilities of Cu and Mg are reduced by Li, and the solid solubilities of Cu and Li are reduced by Mg, thus reducing the age hardening and the UTS obtainable." (Mondolfo, p. 641). Thus, the additional Cu should not be taken into solid solution during solution heat treatment and cannot enhance precipitation strengthening, and the presence of the insoluble theta-phase lowers toughness and corrosion resistance.
One reference that teaches the use of greater than 5 percent Cu is U.S. Pat. No. 2,915,391 to Criner, assigned to Alcoa. The reference discloses Al-Cu-Mn base alloys containing Li, Mg, and Cd with up to 9 weight percent Cu. Criner teaches that Mn is essential for developing high strength at elevated temperatures and that Cd, in combination with Mg and Li, is essential for strengthening the Al-Cu-Mn system. Criner does not achieve properties comparable to those of the present invention, i.e. ultra high strength, strong natural aging response, high ductility at several technologically useful strength levels, weldability, resistance to stress corrosion cracking, etc.
Copending U.S. Pat. application Ser. No. 07/83,333, of Pickens et al, filed Aug. 10, 1987, discloses an Al-Cu-Mg-Li-Ag alloy with compositions in the following broad range: 0-9.79 Cu, 0.05-4.1 Li, 0.01-9.8 Mg, 0.01-2.0 Ag, 0.05-1.0 grain refiner, and the balance Al. Specific alloys within these ranges possess extremely high strengths, which appear to be due in part to the presence of silver-containing precipitates.
Copending U.S. Pat. application Ser. No. 07/233,705 of Pickens et al, filed Aug. 18, 1988, of which this application is a continuation-in-part, discloses Al-Cu-Mg-Li alloys with compositions in the following broad range: 5.0-7.0 Cu, 0.1-2.5 Li, 0.05-4 Mg, 0.01-1.5 grain refiner, and the balance Al. The present invention encompasses the ranges disclosed in the parent application. In addition, the present invention encompasses an embodiment to alloys comprising lower amounts of Cu, i.e. 3.5-5.0 percent, in which the levels of Li and Mg are held within narrow limits. The lower Cu embodiment of the present invention represents a group of alloys which have been found to possess highly improved properties over prior art Al-Cu-Li-Mg alloys. Thus, the present invention encompasses a family of alloys which exhibit improved properties compared to conventional alloys. For example, the present alloys possess improved strengths in both cold worked and non-cold worked tempers. In addition, the present alloys exhibit an extremely strong natural aging response. Further, the alloys have high strength/ductility combinations, low density, high modulus, good weldability, good corrosion resistance, improved cryogenic properties and improved elevated temperature properties.